Field of the Invention
The present invention relates to a method and an apparatus for collecting tags through tag recognition, and more particularly to a method and an apparatus for collecting tags through tag recognition in an active radio frequency identification (RFID)
Description of the Related Art
An RFID technology is a technology where a reader may recognize information on a tag attached to an object in non-contact communication. The RFID tag may be categorized into an active tag and a passive tag according to whether to supply power. The active tag includes a battery therein so that power may be self-supplied. The passive tag use an induction current obtained through electromagnetic induction between the reader and the tag. Since the active RFID tag includes a battery therein, the active RFID tag may be recognized at a wider range and may store more data as compared with the passive RFID tag. The active RFID tag may be recognized even if the active RFID tag is attached to metal. Because of the above characteristics, the active RFID tag may be used in a logistics management system in an airport or a harbor to manage a plurality of objects, and may be variously used in a national defense field and an industrial field.
In the RFID system, readers collect information of tags in a recognition range periodically or when there is necessary information. Such a process of collecting the information refers to tag collection. A time required for tag collection is one important parameter representing performance of an RFID system. When a plurality of tags transmits simultaneously in a collection process, collision may occur during a process of receiving information by a reader. Such collision problem increases a tag collection time. If the number of tags is increased, the collision problem becomes more severe. In order to solve the collision problem, an anti-collision algorithm is used in a tag collection.
There were existing standard technologies with respect to the active RFID system which includes the anti-collision algorithm between tag and reader. The standards define a protocol and the anti-collision algorithm to transmit data between a reader and a tag in an active RFID system operating on a 433 MHz frequency band. In the standards, the reader broadcasts a wake up command such that tags are switched to a preparation state which may respond to the command before tag collection. After transmitting the wake up command, the reader starts a collection round by broadcasting a tag collection command. Since the data in the active tag is more than that in the passive tag, in order to reduce energy consumption due to collision upon transmission of data, a collection round of the active tag is divided by a listen period (LP) and an acknowledge period (AP). The tag collection command includes a window size and the window size represents a length of LP (i.e., the number of slots in LP) from which the tag may select a slot. If tags receive collection command representing start of the tag collection, the tags select an arbitrary slot in the LP period using an anti-collision algorithm based on Aloha to transmit a tag ID thereof. The tags generates a random integer ranging from 1 to the window size, and transmit a tag response including a tag-ID as a response thereof to the collection command in a slot corresponding to the random integer number.
A state of each slot at the LP is divided into three cases according to the number of tag responses at the slot. A first case represents a success slot which receives one response and is successfully recognized. The second case represents a collision slot where collision occurs because a plurality of tags responds. A third case represents an idle slot to which the tag is not responded. If the LP is terminated, the reader starts AP by transmitting a read command in point-to-point correspondence in the order of succeeding the recognition at the LP. If the tag receives the read command, the tag transmits data. If the data is successfully received from the tag, the reader transmits a sleep command in order to reduce power consumption of the tag. The tag switched to a sleep mode through reception of the sleep command does not respond commands from the reader in a next collection round. Tags which are not recognized by the reader pass through other repeated collection round. If one collection round is terminated, the reader estimates a window size to be used at an LP of a next collection round through an LP of a previous collection round. If a new collection round starts, the reader broadcasts the collection command and estimated window size therein. Tags which are not recognized at a previous collection round select an arbitrary slot in the window size to transmit a response.
FIG. 1 illustrates an operation of tag collection of the existing standard. An environment including one reader and five tags within a recognition range is considered. The reader transmits a wake up command such that tags within a recognition range become a preparation state. The reader then transmits the collection command to start LP. Tags receiving the collection command including the window size select a slot within the window size range. In FIG. 1, tag 1 and tag 4 transmit response with a tag-ID thereof without collision. Tag 1, tag 3 and tag 5 simultaneously transmit the response with tag-ID to the same slot so that collision occurs. The reader transmits a read command to the tag 2 and the tag 4 which are successfully recognized. The reader transmits the read command in an order of successful recognition at an LP of the same collection round. The tag 2 receiving the read command transmits a tag-ID and additional data to the reader. If the reader successfully receives the data of tag 2, the reader transmits a sleep command to tag 2 so that the tag 2 can switch to sleep mode and reduce power consumption. In the same manner, the tag 4 is switched to a sleep mode after a data transmission process. The collided tags, i.e., tag 1, tag 3, and tag 5, are not switched to the sleep mode and transmit the response with tag-ID through the same process in a next collection round.
A tag collection method in the standards use point-to-point correspondence at the AP. Therefore, if the number of tags is increased, the control overhead at AP, e.g., the number of read and sleep commands, is also increased. At the AP, since the tag maintains an idle state until the sleep command is received after termination of the data transmission, the energy is consumed unnecessarily.